RNA complexity measurements show that most of the complex hnRNAs are held in common between various tissues in higher eukaryotes as divergent as sea urchins, tobacco and rats. In sea urchins, it has been shown by Lev et al. (Devel. Biol. 76:322, 1980) that messenger sequences for early embryos were present at low copy number, in the nuclear RNA of embryos at all stages of development and in "inappropriate" adult tissues. Our complexity data in rat suggest that various tissues have significant differences in nuclear RNA complexity, but that those sequences expressed in a given tissue are also found in other tissues having a higher absolute complexity. These data support both transcriptional and post-transcriptional mechanisms for generating tissue-specific RNAs. This proposal focuses on the role of post-transcriptional processing in generating tissue-specific mRNAs, particularly in neural tissue which has the greatest diversity of both hnRNA and mRNA sequences. Using cloned probes to known tissue-specific genes (casein, albumin, prostate-specific 22K gene, tyrosine hydroxylase) and probes representing rare, brain-specific or housekeeping transcripts we want to determine if these RNA sequences are present in "inappropriate" tissues, and, if so, at what stage is the production of protein blocked. To do this, we will determine if these sequences are nuclear restricted, polyadenylated, and/or spliced by hybridization of cloned sequences to various RNAs (cytoplasmic, nuclear, poly A+, poly A-, etc.) and to size-fractionated RNA (by Northern analysis). We also plan to compare the transcription rate, and the state of the chromatin encoding sequences from rare versus abundantly transcribed genes. In addition, with clones corresponding to rare, brain-specific mRNAs we will determine if they are developmentally regulated and members of multiple gene families.